The long-term cardiac prognosis of 24 clinically healthy men with complete right bundle branch block (RBBB), identified from the 1,140 men constituting the BLS population, was assessed over a follow-up period averaging 8.4 years. When compared with a control group matched for age at which RBBB appeared, men with RBBB showed no difference in the prevalence of antecedent coronary risk factors or obstructive lung disease. Their incidence of angina pectoris, myocardial infarction, valvular heart disease, cardiomegaly, congestive heart failure, advanced heart block, or cardiac death did not differ from that of the control group over the observation period. Furthermore, at latest follow-up maximal aerobic exercise tolerance and chronotropic response to maximal exercise were not impaired in men with RBBB relative to controls. However, axis deviation leftward of -30 degrees was present in 46 percent of RBBB men but only 15 percent of controls by latest follow-up. Although a PR interval prolongation greater than 40 msec. developed in only 6 percent of control subjects over the observation period, such prolongation occurred in 29 percent of men with RBBB. These results support the concept that RBBB in these asymptomatic men is a manifestation of a primary abnormality of the cardiac conduction system but has no demonstrable adverse effect.